The long range purpose of this project is to investigate the role of type C retroviruses as an etiologic agent and a vector of genetic information for neoplasia, the use of viral mutants to analyze the regulation of gene expression associated with cell differentiation and oncogenesis, and the immunologic responses of the host to tumor associated antigens. The topics of current interest are: 1) Further characterization of amphotropic viruses isolated in this laboratory. 2) Molecular cloning of 11A-MuLV and amphotropic virus DNA by recombinant DNA techniques. 3) Expression of histocompatibility antigens and other differentiation antigens in oncogenic murine trophoblast cell lines. 4) Isolation and characterization of polyoma virus mutants that can grow in trophoblast and/or embryonal carcinoma cells. 5) Further studies on llA-MuLV-induced Meth A-transplantation rejection antigen. 6) Natural killer cell susceptibility of trophoblast cells.